The demand for enantiomerically pure compounds has grown rapidly in recent years. One important use for such chiral, non-racemic compounds is as intermediates for synthesis in the pharmaceutical industry. For instance, it has become increasingly clear that advantages (reviewed in, e.g. , Stinson, S. C. , Chem Eng News, Sep. 28, 1992, pp. 46-79) include fewer side effects and greater potency of enantiomerically pure compounds.
Traditional methods of organic synthesis have often been optimized for the production of racemic materials. The production of enantiomerically pure material has historically been achieved in one of two ways: use of enantiomerically pure starting materials derived from natural sources (the so-called "chiral pool"), or resolution of racemic mixtures by classical techniques. Each of these methods has serious drawbacks, however. The chiral pool is limited to compounds found in nature, so only certain structures and configurations are readily available. Resolution of racemates often requires the use of resolving agents, which may be inconvenient and time-consuming. Furthermore, resolution often means that the undesired enantiomer is discarded, thus wasting half of the material.
Epoxides are valuable intermediates for the stereocontrolled synthesis of complex organic compounds due to the variety of compounds which can be obtained by epoxide-opening reactions. For example, .alpha.-amino alcohols can be obtained simply by opening of an epoxide with azide ion, and reduction of the resulting .alpha.-azido alcohol (for example, by hydrogenation). Reaction of other nucleophiles similarly yields functionalized compounds which can be converted to useful materials. A Lewis acid may be added to act as an epoxide-activating reagent.
The utility of epoxides has expanded dramatically with the advent of practical asymmetric catalytic methods for their synthesis (Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis. Ojima, I., Ed.: VCH: New York, 1993; Chapter 4.1. Jacobsen, E. N. Ibid. Chapter 4.2). In addition to epoxidation of prochiral and chiral olefins, approaches to the use of epoxides in the synthesis of enantiomerically enriched compounds include kinetic resolutions of racemic epoxides (Maruoka, K.; Nagahara, S.; Ooi, T.; Yamamoto, H. Tetrahedron Lett 1989, 30, 5607. Chen, X.-J.; Archelas, A.; Rurstoss, R. J Org Chem 1993, 58, 5528. Barili, P. L.; Berti, G.; Mastrorilli, E. Tetrahedron 1993, 49, 6263.)
A particularly desirable reaction is the asymmetric ring-opening of symmetrical epoxides, a technique which utilizes easily made achiral starting materials and can simultaneously set two stereogenic centers in the functionalized product. Although the asymmetric ring-opening of epoxides with a chiral reagent has been reported, in most previously known cases the enantiomeric purity of the products has been poor. Furthermore, many previously reported methods have required stoichiometric amounts of the chiral reagent, which is likely to be expensive on a large scale. A catalytic asymmetric ring-opening of epoxides has been reported (Nugent, W. A., J Am Chem Soc 1992, 114, 2768); however, the catalyst is expensive to make. Furthermore, good asymmetric induction (&gt;90% e.e.) was observed only for a few substrates and required the use of a Lewis-acid additive. Moreover, the catalytic species is not well characterized, making rational mechanism-based modifications to the catalyst difficult.